Position sensors monitor the position or motion of a first mechanical component relative to a second mechanical component, by producing an electrical signal that varies as a function of the relative position of the two components. The relative speed of the two components can also be determined by taking the time derivative of the position Electrical position sensors are an important part of innumerable products, and are useful for determining the status of various automotive actuations and processes that involve either linear or angular displacement. For example, the position of an adjustable automotive seat can be determined by sensing devices mounted in the movable seat frame and the fixed seat guiding rails. The position and the angular velocity of the automotive engine crankshaft can also be determined by the appropriate placement of position sensing devices.
One prior art position sensor, a contacting position sensor, requires physical contact between a signal generator and a sensing element to produce the electrical signal representative of position. Contacting position sensors typically consist of a potentiometer responsive to the signal generator and mechanically responsive to the component position, such that the output electrical signals vary as a function of the component's position. Motion-induced contact wear limits the durability of the contact-type position sensors.
Non-contact magnetic type position sensors determine position by measuring changes in a magnetic field. Ferromagnetic material disposed on a moving object passes through a constant magnetic field, modulating the field in accordance with the object's position. One example of such a magnetic sensor includes a ferromagnetic target wheel attached to a rotating axle, the speed and/or position of which is to be determined. In one exemplary embodiment the target wheel comprises a plurality of ferromagnetic teeth defining slots therebetween. The constant magnetic field is produced by a stationary biasing magnet (conventionally a permanent magnet) positioned adjacent to the periphery of the target wheel. A magnetic field sensitive device, such as a magnetoresistor is mounted on the stationary magnet for measuring the magnetic field developed by the stationary magnetic, as modulated by the target wheel ferromagnetic teeth. As the wheel rotates the teeth pass adjacent the stationary magnet, changing the reluctance of the magnetic circuit and in turn varying the magnetic flux density of the magnetic field produced by the stationary magnet. These variations are sensed by the magnetoresistor and manifested as variations in the resistance thereof.
Electronic circuitry responsive to the magnetoresistor produces an analog signal that varies in response to the magnetic field flux density variations. Thus a voltage signal in the form of a DC-biased waveform is produced. The waveform characteristics correspond to the shape and spacing of the teeth. When the signal exceeds a predetermined threshold, a tooth in the wheel has been detected adjacent the magnetoresistor. By appropriately spacing the teeth along the target wheel, the angular position of the rotating shaft can be determined. The angular velocity can also be determined as the rate of change of the position. It is known that the resistance of the magnetoresistor, and thus the position accuracy of such a device, is affected by the temperature, the air gap, magnet aging and the positional accuracy of the teeth relative to the rotating shaft.
A Hall effect device can be used in lieu of a magnetoresistor to sense the changing magnetic field and provide an output signal in response thereto. As is known, a Hall effect device comprises a current-carrying conductor that when placed in a magnetic field such that the magnetic field flux lines are perpendicular to the direction of current flow, generates a voltage across the device that is perpendicular to both the direction of current flow and the magnetic flux lines. Thus the Hall effect voltage, which is a function of the magnetic field flux density, serves as a position indicator for a ferromagnetic target.
Whether a magnetoresistor or a Hall effect device is utilized to sense the magnetic field and thus the object position, the position sensor must be accurate, in that it must produce an electrical signal based upon the measured position. An inaccurate position sensor hinders the proper position evaluation and control of the moving component. A position sensor must also be sufficiently precise in its measurement, although the degree of precision required depends upon the specific application. For some applications, only a rough indication of position is necessary. For instance, an indication of whether a valve is substantially opened or closed may be sufficient in some situations. In other applications a more precise indication of the valve position may be required. The position sensor must also be sufficiently durable for the environment in which it is placed. For example, a position sensor used on an automotive engine valve will experience almost constant movement while the automobile is in operation. The position sensor must be constructed of mechanical and electrical components that allow it to remain sufficiently accurate and precise during its projected lifetime, despite considerable mechanical vibrations and thermal extremes and gradients.
The ferromagnetic targets discussed above are typically large and heavy structures, e.g., gears and slotted disks, manufactured by machining, stamping, blanking, powder metal technology, etc. These manufacturing methods are not only expensive, but are also not suitable for manufacturing targets with fine features and complex geometries that are required for high-accuracy small target sensors. Asymmetries in the placement of the teeth in a target wheel or changes in gap distance as the target wheel rotates cause inaccuracies in position determination.
Targets with precise features are particularly needed in state-of-the-art continuous linear and angular position sensors. Such continuous sensors determine position continuously over a range of values, such as angular rotation between 0° to 120°. By comparison, the toothed wheel sensors described above provide discrete position indications when a tooth passes adjacent the field sensing element. The continuous sensors employ a single shaped target where the shape is designed to produce continuous variations in the magnetic field as the target moves relative to the sensor. A spiral shape is one example of a continuous target. Although it is possible to manufacture precise continuous sensors using the prior art techniques of machining, stamping, etc. described above, precision equipment is required and thus the cost for such sensors is high.
One technique for forming precise ferromagnetic sensor targets is described and claimed in the commonly-owned patent application entitled, Method for Forming Ferromagnetic Targets for Position Sensors, filed on Aug. 6, 2002, and assigned application Ser. No. 10/214,047. According to this method, photolithographic techniques allow for the formation of features as small as 0.1 mm by 0.1 mm, and up to about 1 mm thick for use with either discrete target or continuous target sensors. The magnetic field variations caused by targets with these dimensions can be sensed across air gaps in the range of about 0.25 to 0.5 mm, a range that is typical for high-accuracy position and speed sensors employed in most automotive systems.